1. Field of the Invention
The present invention relates to an oscillator and, more specifically, to a crystal oscillator circuit having a regulated capacitive loading and regulated amplifier gain to optimize both startup and steady state operation of the crystal oscillator.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
Within nearly every electronic subsystem is some form of generator that produces cyclical waveforms. The waveform generator is oftentimes referred to as an oscillator. Depending on the application, an oscillator can be used simply as a source of regularly spaced pulses or clock signals. There are numerous types of oscillators available in the marketplace, ranging from simple RC relaxation oscillators to more complex LC oscillators, and also the more stable crystal oscillators. Crystal oscillators use a piezoelectric material such as quartz, where acoustic waves in the crystal are driven by an applied electric field and, in turn, can generate a voltage at the surface of the crystal. The potential across the resonant network is constrained to vary in time according to the electromechanical characteristics of the quartz crystal lattice. This selectivity in frequency can be electrically modeled by a combination of inductance, capacitance, and resistance which correlate to the characteristics of a given quartz crystal. The size and shape of the quartz crystal is fashioned to produce a specific set of these characteristics which determine the periodicity of sinusoidal variation of electrical potential across the resonant network. The quartz thereby operates as a resonator that is pre-tuned to a specific resonant frequency.
In order to initiate and maintain strain on the crystal, crystal oscillators generally include an amplifier coupled across nodes of the crystal. While the least impedance value across the crystal occurs at its resonant frequency, an amplifier that drives the crystal may pull the frequency of the crystal depending on certain performance traits of that amplifier. Moreover, the oscillator can possibly employ tuning capacitors placed on the nodes of the oscillator. The tuning capacitors may also pull the frequency of the crystal. Therefore, a resonating piezoelectric material is formed by electrical circuitry in combination with the frequency selective quartz electromechanical system, which initiates the resonating frequency and maintains that frequency at a pre-defined amplitude.
The capacitors placed on nodes of the oscillator are oftentimes referred to as CXin and CXout. When combined, CXin and CXout place a capacitive load of CL=(CXin×CXout)/(CXin+CXout). As CXin and CXout increase, the loading capacitance CL will also increase thereby increasing impedance on the crystal oscillator. In addition to the capacitive loading, the crystal resonator also has a motional resistance inherent in the material used to form the crystal. The motional resistance of a crystal depends on the amount of power or drive level (DL) dissipated by the crystal. The crystal's motional resistance is larger under startup conditions when the DL is small compared to the motional resistance presented by the crystal to the amplifier under steady-state conditions. This dependency of crystal motional resistance on DL is commonly termed drive level dependency (DLD). A larger negative resistance from the amplifier is required to offset the larger crystal motional resistance at startup due to DLD, and to therefore initiate crystal oscillation.
An amplifier is typically used to overcome DLD of the motional resistance within the crystal and to offset the capacitive loading and resulting impedance on the oscillator nodes. The amplifier provides what is known as “negative resistance” to offset the real part of the impedance looking into the circuit from the crystal, the real part of the impedance comprising the amplifier, feedback resistor, and load capacitors. The real part of the impedance is oftentimes referred to as the effective resistance, or Re, and is represented as Re=−(gmCXinCXout)/((gmC0)2+ω2(CXinCXout+CXoutC0+C0CXin)2) which is calculated from a linear analysis of the oscillator circuit, where gm is the transconductance (or gain) of the amplifier, C0 is the shunt capacitance of the crystal, and ω is the frequency of the oscillation. At startup, the negative resistance from the amplifier needs to be larger than the motional resistance of the crystal
As shown above, the real part of the impedance or resistance of the circuit not only changes with frequency, but also changes based on the load capacitances at the Xin and Xout nodes. For example, at startup of the oscillator when the frequency and amplitude have not reached their steady state, the resistance of the circuit is at its maximum. Moreover, when the load capacitors are large, the resistance of the circuit is also large. As defined herein, “startup” is a condition in which the amplitude output from the oscillator has not reached its targeted value, and the frequency output from the oscillator has also not reached its targeted value. Once the amplitude and frequency have achieved their targets, then the oscillator is said to be performing in a steady state mode of operation.
It would be desirable to introduce an oscillator that can change the amount of negative resistance seen by the resonating crystal depending on whether the oscillator is in startup mode or steady state mode. Therefore, the desired oscillator would represent an improvement over conventional oscillators if such an oscillator can regulate the capacitive loading and the amplifier gain across the oscillator frequency bandwidth, as well as during the startup and steady state operations of that oscillator.